Concrete is made from cement, water, and aggregates, and optionally one or more chemical admixtures. Such chemical admixtures are added to improve various properties of the concrete, such as its rheology (e.g., slump, fluidity), initiation of setting, rate of hardening, strength, resistance to freezing and thawing, shrinkage, and other properties.
In most cases, chemical admixtures are added at the concrete plant at the time of batching. In a “dry batch” plant, the cement, water, aggregates, and chemical admixtures are added from separate compartments (e.g. bins or silos) into the rotatable drum of the ready mix truck, and the ingredients are mixed together. In a “wet batch” or “central mix” plant, all ingredients are combined and fully mixed in a fixed-location mixer, then dumped into the rotatable drum on the truck. A “shrink mix” plant is similar to a “wet batch” or “central mix” plant, with the exception that the ingredients are only partially mixed in the fixed-location mixer, then mixing is completed within the truck mixer.
In a typical dry batch process, the “head water” is first added, followed by the aggregate and cement, and then followed by the “tail water.” The chemical admixture is usually added with the head or tail water. In this way, it is diluted and enough water is present to rinse all chemical admixtures into the mixing drum. In addition, chemical admixture may be added directly on the aggregate as the aggregate is being conveyed to the drum, thus ensuring that all chemical admixtures enter into the drum of the ready mix truck.
The drum of a ready mix truck is an oblong shape with an opening at one end. It is mounted at an angle such that the opening is at the top. Mixing blades or fins are mounted in a helical pattern inside the drum. When the drum is rotated in one direction, the mixing blades push the concrete to the lower end of the drum and cause mixing. When the drum is rotated in the other direction, the mixing blades push the concrete up to and out of the opening. The drum can only be filled partially full with fluid, plastic concrete, because otherwise the concrete will tend to splash out from the truck beyond a certain point.
After batching, the truck moves away from the loading area of the plant and, in the case of dry-batch or shrink mix concrete, completes the initial mixing of concrete, before departing for the jobsite. Frequently, it is desirable to add additional fluid (water or chemical admixture) after the concrete is batched and initially mixed, including up to the time of final discharge at the jobsite. This may be done because some chemical admixtures perform better when added after batching. It is sometimes necessary to add additional fluids to compensate for variations in batching of all ingredients (e.g. too little water added at batching) or changes in concrete properties over time (e.g. loss of flowability and other rheological properties).
It is known to control the “slump” of concrete in ready-mix delivery trucks by using sensors to monitor the energy required for rotating the mixing drum, such as by monitoring the torque applied to the drum by measuring hydraulic pressure and to adjust fluidity by adding fluid into the mixing drum. Fluid dispensing systems are disclosed in U.S. Pat. Nos. 4,008,093, 5,713,663, and U.S. patent application Ser. Nos. 10/599,130, 11/764,832, and 11/834,002, as examples.
Concrete trucks are commonly equipped with water tanks connected by a hose line directed into the drum opening. In this manner, water can be dispensed into the drum under air pressure in the tank or by pump. Such tank dispensing devices are disclosed in U.S. Pat. No. 4,544,275, U.S. Pat. No. 7,842,096 and U.S. patent application Ser. No. 11/955,737, for example.
It is less common for chemical admixture tanks to be mounted on trucks. When such admixture tanks are present, however, the tank is typically connected to the same hose line used to discharge water into the drum. The chemical admixture may be dispensed into the water line under air pressure or by tank to the pump. This is exemplified in U.S. Pat. No. 7,730,903. The present inventors believe that the use of water dispensing equipment is not ideal for the dispensing of liquid chemical admixtures into the concrete mixer drum.
Hence, it is an objective of the present inventors to provide a novel apparatus and method for dispensing both water and liquid chemical admixtures into concrete mixing drums.